A very long straight wire carries a current I. At the instant when...

According to Fleming's left hand rule direction of force is along $y$ axis when is perpendicular to wire: $\vec{F}=$ Q($\vec{V} \times \vec{B}) \mathrm{B}$ due to $\mathrm{I}$ is acting inwards is into the paper, $v$ is along or: \begin{array}{ll} \therefore& F=a+\left|\begin{array}{ccc} \hat{i}& \hat{h} & \hat{k} \\ v & 0 & 0 \\ 0 & 0 & -\mathrm{B} \end{array}\right|\\ \Rightarrow & F=Q+[-j(-v B)+0] \\ \therefore & F=Q v B \hat{j} \end{array} in $y$ direction.